This invention relates to treated carbon black, suitable for use as semi-conductive compositions such as in electrical cables. More particularly, the present invention relates to carbon black treated with a polyethylene glycol, also known as a homopolymer of ethylene oxide, and to semi-conductive compositions including, for example, a polyolefin and the treated carbon black. Such treated carbon blacks are free-flowing, produce low dust levels, are attrition-resistant, are easily dispersible in polymeric systems and provide enhanced rheological, mechanical and electrical properties. The present invention also relates to a process for producing such treated carbon blacks, and compositions produced therefrom.
Any of a wide variety of carbon blacks may be used in the present invention. Thus the term carbon black is used herein in its generic sense to include types of finely divided carbon such as lamp black, channel black, furnace black, acetylene black and the like. Preferably, the carbon black is in the fluffy form. Although the carbon blacks are easily dispersed in liquid and polymeric systems in their fluffy forms, they are extremely difficult to handle with respect to conveying and accurate weighing. This is particularly due to their low bulk densities and generally high dust levels, which tend to lead to compaction and consequent irregular feed in continuous compounding operations. Such effects are undesirable because, for example, continuous compounding operations are desirable for wire and cable compounds as a means to ensure uniformity and cleanliness of the compounds.
A persistent problem with carbon black has been the relative difficulty and inconvenience associated with manufacturing, transporting and using the carbon black. When the carbon black is packaged, transported and removed from its packaging, carbon black dust is produced.
To improve the handling characteristics of the fluffy carbon blacks, they are generally agglomerated by various mechanical processes to produce pellets, either in the dry state or with the aid of a liquid pelletizing aid. Generally the carbon black particles are held together by weak forces. The most common process is to pelletize the fluffy carbon blacks using a liquid pelletizing aid such as oil or water. However, the agglomeration or densification process has been found to have a detrimental effect on the dispersion characteristics of the fluffy carbon blacks. That is, as the fluffy carbon blacks are agglomerated into pellets, they become less easily dispersible in polymeric systems. Therefore, there exists a tradeoff between acceptable handling characteristics and ease of dispersion.
In order to compound well dispersed formulations with these carbon blacks, they are often pelletized with materials such as sodium ligno sulfonate, water, sucrose, etc., as described below. However, the pelletizing aids often produce less than adequate results, or result in the carbon black being incompatible with the formulation in which it is to be combined. For example, some carbon blacks do not develop adequate pellet strength when pelletized with water. This is due to the very low van der Waals forces existing between the carbon structure. Thus, although in pellet form, they are quite friable and result in high dust levels. The result is inconsistent feed and consequent inhomogeneous dispersion in a compound.
Processes for pelletizing carbon blacks to produce carbon black pellets are known in the art. For example, U.S. Pat. No. 2,065,371 to Glaxner describes a wet pelletization process whereby the fluffy carbon black and a liquid such as water are combined and agitated until spherical carbon black beads are formed. The beads are then dried to reduce the water content to below 1% to form carbon black pellets.
In addition to water, a wide variety of binder additives are known to be useful in the wet pelletization process to further improve the pellet handling characteristics of the fluffy carbon blacks. For example, the following references describe the use of various binder additives as pelletizing aids for producing carbon blacks: U.S. Pat. No. 2,427,238 to Swart describes the use of a number of hygroscopic organic liquids or solutions, including ethylene glycol, in the compounding of rubber-like materials. U.S. Pat. No. 2,850,403 to Day discloses the use of carbohydrates such as sugar, molasses, soluble starches, saccharides and lignin derivatives as pelletization binder additives in the range of 0.1% to 0.4% by weight of the dry carbon black. The wet pellets are then dried for a given residence time at a temperature of from 150.degree. C. to 425.degree. C. to carbonize the carbohydrate binder. U.S. Pat. No. 2,908,586 to Braendle et al. discloses the use of a rosin emulsion as an alternative to the carbohydrates, in a preferred amount of from 0.5% to 2.0% by weight of the dry carbon black. U.S. Pat. No. 2,639,225 to Venuto discloses the use of sulfonate and sulfate anionic surfactants as pelletizing aids in an amount of 0.1% to 0.5% by weight of the dry carbon black. U.S. Pat. No. 3,565,658 to Frazier et al. discloses the use of a fatty amine ethoxylate nonionic surfactant as a pelletizing aid wherein the fatty amine ethoxylate has a level of ethoxylation in the range of from 2 to 50 moles of ethylene oxide per fatty amine group. The nonionic surfactant is disclosed as preferably being present in the range of 0.05% to 5.0%. Similarly, U.S. Pat. No. 3,645,765 also to Frazier et al. discloses the use of a fatty acid or rosin acid ethoxylate nonionic surfactant in the range of 0.1% to 10.0% by weight of the carbon black. The nonionic surfactant is disclosed as having a level of ethoxylation of from 5.0 to 15.0 moles of ethylene per acid group. Soviet Union Patent Publication No. 937,492 discloses the use of a 0.1% to 5.0% aqueous solution of a reaction product of urea and an ethoxylated alkylolamide, where the level of ethoxylation is from 1.0 to 7.0 moles of ethylene oxide per alkylolamide. Finally, U.S. Pat. No. 3,844,809 to Murray discloses the use of a nonionic surfactant containing randomly repeating poly(ethylene oxide) and poly(dimethyl silicone) groups. The reference discloses that 0.4% to 2.5% of an aqueous solution containing 0.001% to 0.1% of the nonionic surfactant results in a reduction in pellet dust levels. Molasses is also included at substantially higher concentrations of up to 2.0% as a co-binder, and nitric acid is included in an amount of up to 15.0% as an oxidizing source. The above patents disclose improved pellet handling qualities, but do not disclose changes in the performance properties of the pelletized carbon black in final product applications.
Among the handling characteristics of the carbon black that may be improved by binder additives and the level of such additives being used in the pelletizing process are such characteristics as adhesion, dispersibility, dispersion rate, viscosity stability and anti-static properties. For example, Japanese Patent Publication No. 01-201,369 discloses the use of a carboxylic acid type amphoteric surfactant in a concentration of from 0.001% to 0.1% in the pelletizing water to produce carbon black pellets with low adhesion and excellent dispersibility. U.S. Pat. No. 3,014,810 to Dybalski et al. discloses the benefits of wet pelletizing a range of pigments, including carbon black, with 0.05% to 5% by weight of a blend of a quaternary ammonium compound and a bis(2-hydroxyethyl)alkyl amine. The disclosed benefits include improvements in dispersion rate, viscosity stability and anti-static properties.
As described above, oil has also been used, with or without the inclusion of water, as a pelletizing aid. For example, U.S. Pat. Nos. 2,635,057 to Jordan, 3,011,902 to Jordan and 4,102,967 to Vanderveen et al. disclose the use of oil, such as mineral oil, in the pelletizing process to improve the handling characteristics of carbon black pellets. Additionally, the use of polymers in an emulsion, organic solvent, solution or molten form has been disclosed as a means of modifying the pellet properties of carbon black, for example as described in U.S. Pat. Nos. 2,511,901 to Bunn (latex emulsions), 2,457,962 to Whaley (aqueous emulsions or dispersions of rubber), 4,440,807 to Gunnell (molten rubber or a solution or emulsion of rubber), 4,569,834 to West et al. (emulsion of an oxidized polyethylene) and 5,168,012 to Watson et al. (a rubber latex) and in Japanese Patent Publication No. 77-130,481.
Other alternative pelletizing aids for producing carbon black pellets include sodium ligno sulfonates, silanes, sucrose, and nonionic dispersants such as alkyl succinimides and alkylated succinic esters. However, such alternatives have not produced favorable carbon black pellets and/or polymer compositions. For example, carbon blacks produced using sodium ligno sulfonates are generally considered unsuitable for use in semi-conductive polymer compositions due to the increased propensity for water tree formation resulting from the increased sulfur content. Other drawbacks of the alternative pelletizing aids include adhesion of the carbon blacks to processing equipment, difficulty in applying the pelletizing aids to the carbon blacks, and (particularly in wire and cable formulations) water tree formation in the polymer compositions.
The use of polyethylene glycol in the production of rubber and thermoplastic resin materials is also generally known. For example, U.S. Pat. No. 4,230,501 to Howard et al. describes the use of polyethylene glycol in a pigment concentrate that is easily dispersed in plastics. The polyethylene glycol or a hydrocarbon resin is incorporated as a viscosity control additive in a natural, petroleum or synthetic wax, which is then mixed with 51% to 85% by weight of a pigment to form a pigment concentrate. U.S. Pat. No. 4,397,652 to Neumann discloses the production of powdered compositions containing organic dyes and optical brighteners, that produce no negligible dust levels. Polyethylene glycol is disclosed both as an adhesive component, at molecular weights greater than 3,000, and as a dust-binding agent, at molecular weights between 200 and 1,000. British Patent Specification No. GB 975,847 discloses the use of an aqueous solution of polyethylene glycol or an aliphatic derivative as a means of producing agglomerates of organic rubber chemicals. Pellets of the composition are formed via an extrusion process, and subsequently dried at low temperatures.
Polyethylene glycol is also known in the art as an additive for direct compounding into crosslinked and thermoplastic resin compositions. For example, U.S. Pat. No. 4,013,622 to DeJuneas et al. describes the incorporation of 100 to 600 ppm of polyethylene glycol having a molecular weight of from 600 to 20,000 into a major amount of low density polyethylene. The polyethylene glycol is incorporated into the thermoplastic resin to reduce the breakdown of the polyethylene during blown film operations. As a further example, U.S. Pat. No. 3,361,702 to Wartman et al. discloses the use of polyethylene glycol or branched ethoxylate molecules as plasticizers for ethylene-acrylic acid co-polymers.
Polyethylene glycol is also known to be useful in the production of polymer compositions. For example, U.S. Pat. Nos. 4,812,505 to Topcik, 4,440,671 to Turbett and 4,305,849 to Kawasaki et al. disclose the use of polyethylene glycol, having a molecular weight of from 1,000 to 20,000, for reducing the water treeing characteristics in polymer compositions for electrical insulation materials. U.S. Pat. No. 4,812,505 to Topcik discloses incorporating from 0.1 to 20% by weight of polyethylene glycol into a polymer composition. U.S. Pat. No. 4,440,671 to Turbett discloses the incorporation of from about 0.2 to about 1 part of polyethylene glycol, having a molecular weight of from 1,000 to 20,000, per part by weight of diphenyl amine. U.S. Pat. No. 4,305,849 to Kawasaki et al. discloses the incorporation of between 0.3% and 10% by weight of polyethylene glycol directly into an insulating polymer composition by kneading the polyethylene glycol with the polymer.
In this respect, water treeing refers to a phenomenon that occurs when a polymeric insulation material such as polyolefin is subjected to an electrical field over a long period of time in an environment containing water. This phenomenon is to be distinguished from electrical trees (carbonization of the insulation material due to electrical discharges) and chemical trees (crystals formed from reactive gases on the conductor surface).
The reduction of the water treeing phenomenon is also addressed in U.S. Pat. No. 4,612,139 to Kawasaki et al., which is directed to reducing the water treeing problem in semi-conductive polymer compositions containing carbon black. The patent discloses that polyethylene glycol can be directly incorporated into a semi-conductive polymer composition to eliminate the water treeing phenomenon. Polyethylene glycol, having a molecular weight of from 1,000 to 20,000, is incorporated into the polymer in an amount of from 0.1% to 20% by weight of the polymer. Similar compositions are disclosed in German Patent No. DE 27 23 488. The German patent discloses that polyethylene glycol and other mobile additives are beneficial in reducing the interlaminar adhesion between the insulation layer and the outer conductive layer (e.g., the insulation shield) in an electrical cable construction.
Japanese Patent Publications Nos. 61-181,859 and 61-181,860 disclose electroconductive compositions. The compositions comprise a crystalline polyalkylene oxide and carbon black or graphite. 61-181,859 also discloses that the polymer is modified to contain carboxyl or carboxylic acid side chains.